Synthetic Studies toward GKK1032s, Novel Antibiotic Antitumor Agents: Enantioselective Synthesis of the Fully Elaborated Tricyclic Core via an Intramolecular Diels−Alder Cycloaddition

An enantioselective synthesis of the fully elaborated tricyclic decahydrofluorene core (ABC-ring system) of GKK1032s, novel antimicrobial and antitumor agents, has been accomplished for the first time by employing a highly diastereoselective intramolecular Diels−Alder (IMDA) reaction. The key substrate for the IMDA reaction was efficiently prepared through (i) an intermolecular Diels−Alder reaction between a siloxydiene and an optically active enone derived from d-mannitol to construct the appropriately functionalized C-ring and (ii) CuCl-promoted Stille coupling of an (E)-vinyl iodide and a vinylstannane to install the requisite triene side chain as the crucial steps.